iio: adc: palmas_gpadc: use module_platform_driver to simplify the code

module_platform_driver() makes the code simpler by eliminating
boilerplate code.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Link: https://lore.kernel.org/r/20200914065401.3726354-1-liushixin2@huawei.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>